The invention relates to a process for improving the quality of gasoline obtained by catalytic cracking.
Gasoline produced by catalytic cracking has a high olefins content, which is the reason why it shows a tendency to gumminess. Furthermore gasoline produced by catalytic cracking has a relatively low aromatics content, which is the reason whey it has a relatively low octane number. On account of the aforementioned properties it is preferred to subject gasoline obtained by catalytic cracking to quality improving treatment before using it as a mixing component for motor gasoline. This improvement of the quality may be performed by catalytic reforming in which, among other things, the olefins content decreases and the aromatics content increases. A drawback to the catalytic reforming of gasoline produced by catalytic cracking is its high content of sulfur and nitrogen compounds. Since the presence of sulfur and nitrogen compounds in the feed has a very unfavorable influence on the performance of reforming catalysts, said compounds must be removed from the feed to a considerable extent before the latter can be subjected to catalytic reforming. This requires a catalytic hydrotreatment under extra severe conditions. Although the above-mentioned two-step process in which the gasoline is first subjected to a catalytic hydrotreatment under extra severe conditions and then to catalytic reforming results in a substantial betterment of the gasoline produced by catalytic cracking, there is an urgent need for a process which leads to the object aimed at in a single step.
Recently, novel crystalline metal silicates of a special structure were synthesized which show catalytic activity in the conversion of non-aromatic organic compounds, such as olefins, into aromatic hydrocarbons. The catalytic performance of these silicates is to a great extent insusceptible to the presence of sulfur and nitrogen compounds in the feed. The crystalline metal silicates concerned are characterized in that after one hour's calcination in air at 500.degree. C. they have the following properties:
(a) an X-ray powder diffraction pattern in which the strongest lines are the four lines mentioned in Table A,
TABLE A ______________________________________ d (.ANG.) ______________________________________ 11.1 .+-. 0.2 10.0 .+-. 0.2 3.84 .+-. 0.07 3.72 .+-. 0.06, and ______________________________________
(b) have a composition which, in addition to SiO.sub.2, includes Fe.sub.2 O.sub.3 and/or Al.sub.2 O.sub.3, and in which the SiO.sub.2 /(Fe.sub.2 O.sub.3 +Al.sub.2 O.sub.3) molar ratio is higher than 10.
An investigation into the use of the above-mentioned crystalline iron, aluminum, and iron/aluminum silicates as catalysts for improving the quality of a complete gasoline fraction such as it occurs in a product obtained by catalytic cracking has shown that although, generally, by using crystalline silicates belonging to this group a reduction of the olefins content as well as a rise in the aromatics content can be achieved, there are a number of cases in which the results obtained are not good enough to decide to use them on a technical scale. The investigation has shown that, on account of the activity, selectivity and stability the abovementioned crystalline metal silicates exhibit when used for improving the quality of a complete gasoline fraction as it occurs in a product obtained by catalytic cracking, they can be divided into three classes.
Class I comprises the iron silicates and iron/aluminum silicates having a SiO.sub.2 /Fe.sub.2 O.sub.3 molar ration lower than 250 and a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of at least 500.
Class II comprises the aluminum silicates and iron/aluminum silicates having a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio lower than 500.
Class III comprises the iron silicates, aluminum silicates and iron/aluminum silicates having a SiO.sub.2 /Fe.sub.2 O.sub.3 molar ratio of at least 250 and a SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio of at least 500.
The crystalline metal silicates belonging to class I have sufficient activity, selectivity and stability and are therefore suitable for use such as they are. The crystalline metal silicates belonging to class II have sufficient activity and selectivity, but insufficient stability. The crystalline metal silicates belonging to class II have sufficient activity and selectivity, but insufficient stability. The crystalline metal silicates belonging to class III have insufficient activity. The crystalline metal silicates belonging to classes II and III cannot therefore be used such as they are.
Further investigation into this subject has resulted in three measures which have an extremely favorable influence on the selectivity or the stability of the catalysts mentioned hereinbefore. The invention relates to the application ot these measures (hereinafter referred to as measures 1-3) either individually or combined. Since these measures are unable to improve the activity of the catalysts mentioned hereinbefore, the metal silicates belonging to class III, which have insufficient activity, will not be considered here.
Measure 1 relates to the use of a mixture of two catalysts, one of which is the crystalline metal silicate. Measure 1 leads to considerable enhancement of the selectivity. Since measure 1 is unable to improve the stability of the abovementioned catalysts, this measure when used individually, is only suitable for metal silicates belonging to class I. These metal silicates show sufficient selectivity in themselves, but the use of measure 1 adds still considerably further to the selectivity of these metal silicates.
Measure 2 relates to the use of the metal silicate with a heavy fraction, which fraction has a give aromatics content and has been separated by distillation from the complete gasoline fraction present in the product obtained by catalytic cracking. Measure 2 leads to considerable enhancement of the selectivity. Since measure 2, like measure 1, is unable to improve the stability of the aforementioned catalysts, this measure when used individually, or optionally together with measure 1, is only suitable for metal silicates belonging to class I. As remarked before, these metal silicates show sufficient selectivity in themselves, but the use of measure 2 adds still considerably to their selectivity and it may be enhanced even further, if desired, by combining measure 2 with measure 1.
Measure 3 relates to the use of the metal silicate with a mixture of a light and a middle fraction, which mixture has a given aromatics content and has been separated by distillation from the complete gasoline fraction present in the product obtained by catalytic cracking. Measure 3 leads to such an improvement of the stability that metal silicates belonging to class II, which as such have insufficient stability, are now suitable for use. Measure 3 is also suitable for metal silicates in themselves, but the use of measure 3 adds still considerably to their stability. If desired, measure 3 can be combined with measure 1 and/or measure 2, which, in addition to enhancement of stability, results in considerable enhancement of selectivity--when measure 1 or 2 is used--or very considerable enhancement of selectivity--when both measures 1 and 2 are used.
The present patent application forms the subject matter of Netherlands patent application No. 8302520 (K-5691) and relates to the application of measure 1 when metal silicates belonging to class I are used. The application of measure 2 when using metal silicates belonging to class I forms the subject matter of Netherlands patent application no. 8302519 (K-5692). The application of measure 3, optionally combined with measure 2, when metal silicates belonging to classes I and II are used, forms the subject matter of Netherlands patent application No. 8302518 (K-5704).